This invention relates to the inner stripe with self-aligned saturable-absorber (IS.sup.3) laser, which is characterized by its low noise operation.
Semiconductor lasers are employed in home-use optical-data processing systems such as digital audio disks (DAD) and video disks (VD). Semiconductor lasers enable the reduced size and cost and the marketability of these systems.
However, the noise characteristics of semiconductor lasers has required improvement before they could be ideally utilized in these systems. Noise suppression is essential for the VD players, which read analog signals, while DAD players, which read digital signals, have higher noise allowances. Noise suppression, however, also becomes critical in DAD players as their sizes decrease.
Relative-intensity-noise (RIN) levels of less than 5.times.10.sup.-14 Hz.sup.-1 and 1.times.10.sup.-11 Hz.sup.-1 are required in analog and digital playback systems, respectively. RIN is defined as RIN=(.DELTA.P/P).sup.2 /.DELTA.f, where P is the average output power and .DELTA.P is the noise power as measured in band .DELTA.f around the center frequency fo.
Single-longitudinal-mode index-guided lasers suffer from mode-hopping noise, which is caused by ambient temperature, fluctuations and optical feedback noise, which is induced by the light reflecting back to the emitting facet. Thus, RIN is so high (.about.10.sup.-11 Hz.sup.-1 around fo=200 KHz) that single-longitudinal-mode lasers are generally not used even in digital playback systems.
On the other hand, multi-longitudinal-mode gain-guided lasers are less sensitive to the temperature fluctuations and the light feedback because not all the longitudinal modes but a few are sensitive at the same time. Therefore, gain-guided lasers are preferably used in digital playback systems such as DAD players. However, competition among their longitudinal modes increases RIN as high as 1.times.10.sup.-13 Hz. Therefore, gain-guided lasers cannot be used for analog playback systems such as VD players.
In FIG. 1, the oxide-stripe laser is depicted as an example of a gain-guided laser. The oxide-stripe laser comprises an n-type GaAs substrate 2, an n-type Al.sub.x Ga.sub.1-x As cladding layer 4, an n- or p-type Al.sub.y Ga.sub.1-y As (0.ltoreq.y&lt;x.ltoreq.1) active layer 6, a p-type Al.sub.x Ga.sub.1-x As cladding layer 8, a p-type GaAs ohmic layer 10, a SiO.sub.2 insulating layer 12 and metal electrodes 14 and 16. A pumping current is injected into the active layer 6 through the stripe window opened in the insulating layer 12. Thus, optical gain is restricted along the junction plane and single filament operation is realized. The oxide-stripe laser, as well as other gain-guided lasers, has a rather long astigmatic distance of about 20 .mu.m or more. Therefore, some compensation optics are required when gain-guided lasers are utilized in optical playback systems.
FIG. 2 shows the channeled substrate planar (CSP) laser, which is disclosed in Japanese Patent Publication (Kokoku) No. 54-5273, as an example of index-guided lasers. The CSP laser comprises an n-type GaAs substrate 2 provided with an etched channel 18, an n-type Al.sub.x Ga.sub.1-x As cladding layer 4, an n- or p-type Al.sub.y Ga.sub.1-y As (0.ltoreq.y&lt;x.ltoreq.1) active layer 6, a p-type Al.sub.x Ga.sub.1-x As cladding layer 8, a p-type GaAs ohmic layer 10, a SiO.sub.2 insulating layer 12, and metal electrodes 14 and 16. In the region outside the channel 18, the n-type cladding layer 4 is thin enough for the optical field to penetrate the lossy substrate 2 and, therefore, an effective refractive index difference is provided. Thus, the optical field is confined to the channel region. The CSP laser, as well as other index-guided lasers, has short astigmatic distance of less than 5 .mu.m. Therefore, for no compensation optics are necessary, using the CSP laser in optical playback systems. However, the CSP laser exhibits such a high value of RIN that it is hardly used, as described earlier.
Japanese Patent Disclosure (Kokai) No. 57-159084 discloses a V-channeled-substrate inner-stripe (VSIS) laser in which an n-type GaAs layer grown on p-type GaAs substrate acts as a current-blocking layer (CBL). The basic structure of this prior art is shown in FIG. 3. The VSIS laser comprises a p-type GaAs substrate 18, an n-type GaAs layer 20, which act as a CBL, a channel 34 etched into the CBL 20 to reach the substrate 18, a p-type Al.sub.x Ga.sub.1-x As cladding layer 22, a p- or n-type Al.sub.y Ga.sub.1-y As (0.ltoreq.y&lt;x.ltoreq.1) active layer 24, an n-type Al.sub.x Ga.sub.1-x As cladding layer 26, an n-type GaAs ohmic layer 28, and metal electrodes 30 and 32. The index-guiding mechanism of the VSIS laser is similar to that of the CSP laser. In the VSIS laser, however, the current is also restricted in the channel 34 because of the CBL, while it is not restricted in the CSP laser. As a result, the VSIS laser has intermediate characteristics between a gain-guided laser and an index-guided laser, namely, multi-longitudinal-mode operation at low power level and rather short astigmatic distance (5.about.15 .mu.m). Due to these favorable characteristics, the VSIS laser is preferably used in DAD players. However, the multi-longitudinal-mode VSIS laser generally exhibits higher value of RIN than required in analog playback systems such as VD players.